Just updating as I made a post over on the E83 forum about opinions on
NRF vs Mahle radiators.
Back to this particular issue, I never could locate a new coolant hose - however I ended up purchasing a number of different o-rings from a speciality o-ring supplier (Hydraulic Seals Australia, hsa.com.au) if you're in Australia, and the 22.4mm ID 3.5mm thick o-ring seals well on that connection.
I have a bit of a reference going for M57 engine cooling system o-rings:
There are 3 sizes on the coolant pipe for the heater return-hose (
https://www.realoem.com/bmw/enUS/sho...diagId=11_3878 - part number 1)
IDxThickness [mm]:
1. 16x3 - Same as smaller connection on upper radiator tube.
2. 22.4x3.5 - Same as the connection I had an issue with above.
3. 26x4
The larger part of coolant flange (and same as the larger connection on the upper radiator hose):
4. 38x4
If you ever have trouble with a coolant pipe and can measure the Male side OD, and it matches up to one of these o-ring IDs, it's likely the correct one.